A strain gauge type acceleration sensor 30 of the bulk type shown in FIG. 12 is well known as an acceleration sensor used for ABS (the anti-lock brake system), the air bag system and the navigation system used for an automobile.
This type acceleration sensor 30 is manufactured in such a manner that a bulk of the single crystal base plate 31 made of silicon, the shape of which is rectangular parallelepipedic, and the crystal plane index of which is (100), is selectively etched from the front and reverse sides, that is, a bulk of the single crystal base plate 31 is subjected to crystal anisotropy etching. A detecting portion 32 formed by etching is composed of a mass portion 33 which functions as a weight, and a cantilever 34 for supporting the mass portion so that the mass portion can be displaced. A strain gauge composed of a plurality of diffusion strain resistances 35 is formed on an upper face of the cantilever 34. Accordingly, when an acceleration is impressed upon this acceleration sensor 30, the mass portion 33 is displaced in a predetermined direction, so that the cantilever 34 is bent. At this time, strain is induced in the diffusion strain resistance 35 provided on the cantilever 34. As a result, by the piezo-resistance effect of silicon, a resistance value of the diffusion strain resistance 35 is increased or decreased. When a change in the resistance value is detected, the acceleration can be found.
In this connection, in order to obtain a predetermined detection sensitivity by the conventional acceleration sensor 30, it is necessary to use a single crystal base plate 31 made of silicon, the thickness of which is at least 200 to 300 mm so that a thick mass portion 33 can be formed.
However, when the thick single crystal base plate 31 of silicon is etched from the reverse side, consideration must be given to the anisotropic etching characteristic, that is, consideration must be given to that an etching hole, the opening angle of which is q=125.26.degree., is formed along a plane of (111). Therefore, it is necessary to set the size of an opening portion "a" at a fairly high value. Due to the foregoing, width W of the chip is increased. Therefore, it is difficult to sufficiently reduce the dimensions of the overall acceleration sensor 30.
As a different type sensor from the above acceleration sensor 30, there is provided a surface and capacitance type acceleration sensor which is manufactured when a thin film formed on the front side of a silicon base plate is etched. In this type acceleration sensor, in a recess portion formed on the front side of the silicon base plate, there is provided a mass portion, which composes a detecting portion, supported by a support structure in such a manner that the mass portion can be displaced. On an upper surface of the mass portion, there are provided a plurality of movable electrodes. There are provided a plurality of stationary electrodes at positions respectively corresponding to the movable electrodes, wherein a predetermined distance is maintained between the stationary electrodes and the movable electrodes. Accordingly, when an acceleration is impressed upon this acceleration sensor, the mass portion is displaced. In accordance with the displacement of the mass portion, capacitance of a condenser formed by each movable and stationary electrode is changed. When the change in capacitance is detected, a direction and intensity of the impressed acceleration can be detected.
However, when the recess portion is formed on the surface of the silicon base plate, it is necessary to previously form a sacrifice layer because etching of sacrifice layer must be conducted. Therefore, the manufacturing process becomes complicated. Since it is common to support the mass portion with a cantilever in the case of a conventional surface and capacitance type acceleration sensor, the support structure tends to be damaged when a high intensity of acceleration is impressed upon the mass portion.